1. Field of the Invention
The present invention is directed to an X-ray exposure system of the type having an X-ray apparatus with an X-ray source and an X-ray receiver that are adjustable relative to the subject for registering successive 2D projections of a subject from different projection directions, and means for reconstructing 3D images of the subject from the 2D projections.
2. Description of the Prior Art
X-ray exposure systems of this type usually have an X-ray apparatus with a C-arm for carrying the X-ray source and the X-ray receiver, this C-arm being seated in a holder at the X-ray apparatus so as to be motor-adjustable in a specific angular range along its circumference (orbital motion). For acquiring 2D projections from different projection angles for 3D image reconstruction of, for example, a body region of a subject with the C-arm X-ray apparatus, the C-arm, after appropriate placement relative to the subject to be examined, is adjusted along its circumference during the registration of 2D projections of the body region of the subject. 3D images of the body region of the subject are subsequently reconstructed from the 2D projections registered with the X-ray apparatus during the adjustment motion of the C-arm. The reconstruction of 3D images, however, assumes exact knowledge of the projection geometry, i.e. the exact knowledge of the position of the X-ray source and the X-ray receiver and of the projection angle during each of the individual 2D projections.
Known C-arm X-ray apparatuses exhibit mechanical instabilities particularly relating to the adjustment of the C-arm along its circumference, as a result of which deviations of the real adjustment motion of the C-arm from the ideal adjustment motion occur. The determination of the projection geometries is thereby often affected by errors, the quality of the 3D images reconstructed from the 2D projections suffering therefrom.
German OS 195 12 819, for example, discloses an X-ray computed tomography apparatus for 3D imaging that has an X-ray source that emits a conical X-ray beam penetrating a measurement field. A subject to be examined that is arranged in the measurement field is penetrated by the X-ray beam, which strikes a planar detector whose output signals are supplied to a computer for the reconstruction of 3D images of the subject. For determining the projection geometries of the X-ray source and of the detector at the point in time of the individual 2D projections, two rings provided with metal structures are provided, these being arranged above and below the region of the subject to be examined. The metal structures of the rings are visible in the 2D projections of the body region to be examined, so that the respective projection geometries of the 2D projections can be calculated from their position.
This method for determining the projection geometries, however, has the disadvantage that the rings have a relatively large diameter, so that the spacing between the X-ray source and the rings is very small (a few centimeters). The metal structures therefore appear greatly enlarged in the 2D projections, so that large parts of the 2D projections are overlaid by the metal structures. Further, only a small region of the metal structures of the rings is imaged in the 2D projections, so that the determination of the projection geometries is difficult on the basis of the slight number of imaged metal structures.
U.S. Pat. No. 5,109,397 discloses a mobile computed tomography apparatus having an X-ray arrangement rotating around a rotation center and comprising an X-ray source and an X-ray receiver. Sensors are allocated to this X-ray arrangement, these sensors moving along with the X-ray arrangement and interacting with a stationary ring allocated to the rotation center for detecting lateral movements of the X-ray arrangement during a scan. The sensors thereby generate signals whose evaluation allows the spacings between their defined point of attachment and the ring to be determined. The acquired data are subsequently utilized in the reconstruction of tomograms. The ring is thereby arranged in the propagation path of an X-ray beam emanating from the X-ray source. In another embodiment of this computed tomography apparatus, the ring can be provided with X-ray-positive marks that are imaged in the 2D projections, whereby their interpretation enables the determination of the projection geometries.
German OS 36 04 955 discloses an X-ray diagnostic apparatus having an image generating system with an X-ray radiator and radiation receiver as well as a patient table. Position sensors in the form of potentiometers that acquire the position of adjustable components of the image generating system are connected to these components. The potentiometers, however, are not suited for the exact determination of the projection geometries.
German OS 195 35 583 also discloses an X-ray diagnostic apparatus with a positioning aid. A light transmitter for emitting a light beam is provided at an X-ray image intensifier so that this light beam is focused onto a X-ray radiator lying opposite the X-ray image intensifier. In this way, a positioning of the X-ray radiator and the X-ray image intensifier can ensue with reference to an examination subject without emitting X-rays.